A device of this kind is known from DE-OS 35 29 108 which corresponds to U.S. Pat. No. 4,752,944 as well as from the references cited therein. The two-dimensional charge pattern which is generated on the surface of the photoconductor and which essentially corresponds to the spatial distribution of the X-ray intensity is scanned by an electrometer device. The electrometer device comprises a plurality of electrometer probes which are aligned in a row. During the scanning of the charge pattern, a first line of the charge pattern is first scanned. Subsequently, the photoconductor is displaced relative to the electrometer probes, in a scanning direction extending perpendicular to the row direction, over approximately the width of one line. Subsequently, the second line is scanned. This operation is repeated until the entire charge pattern of the photoconductor has been scanned. The electric signals generated during the scanning by the electrometer probes are amplified and are available for further signal processing.
In devices of this kind a problem is encountered in that the individual electrometer probes only rarely have the same sensitivity, so that the individual probes supply signals of different intensity. As a result, streaks occur in the image formed after complete scanning of the charge pattern. Similar problems occur due to differences in the distances between the electrometer probes and the photoconductor at the area of the various scanning lines. Exact displacement of the photoconductor relative to the electrometer probes so that an absolutely identical distance occurs between the electrometer probes and the surface of the photoconductor for all scanning lines is hardly feasible in practice, notably considering the size and the weight of such a photoconductor. These problems also give rise to streakiness of the image formed. Streaks can also be produced in the image due to low-frequency noise in amplifiers succeeding the probes.